Chargaff, in his experiments on the chemistry of DNA estimated the base composition of human DNA and found Adenine constitute 31% and Guanine 19%. The quantity of cytosine in the DNA of a human somatic cell is likely to be

Options

A. `31%`
B. `19%`
C. `62%`
D. `38%`

Answer

To determine the quantity of cytosine in human DNA based on Chargaff's findings, we can follow these steps:

### Step 1: Understand Chargaff's Rules
Chargaff's rules state that in DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). This means:
- A = T
- G = C

### Step 2: Identify Given Values
From the problem, we know:
- Adenine (A) = 31%
- Guanine (G) = 19%

### Step 3: Calculate Thymine (T)
Since adenine pairs with thymine, we can conclude:
- Thymine (T) = 31% (because A = T)

### Step 4: Calculate Cytosine (C)
Since guanine pairs with cytosine, we can conclude:
- Cytosine (C) = 19% (because G = C)

### Conclusion
The quantity of cytosine in the DNA of a human somatic cell is likely to be **19%**.

### Summary of Options
- Option A: 31% (Incorrect)
- Option B: 19% (Correct)
- Option C: 62% (Incorrect)
- Option D: 38% (Incorrect)

Thus, the correct answer is **Option B: 19%**.

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